The present invention relates to an anti-locking hydraulic brake system comprising a master brake cylinder connected, through brake conduits, to wheel brakes, further comprising a separating valve for blocking the brake conduit, a return conduit containing an outlet valve connecting the wheel brake to the intake side of a pump, a pressure conduit connecting the pressure side of the pump to the brake conduit between the separating valve and the wheel brake.
A brake system of the aforementioned type is disclosed, for example, by U.S. Pat. No. 4,636,009. In the system shown therein, the pressure in the wheel brakes can be increased, lowered or maintained constant, through a 3-way/3-position valve that blocks or releases the brake conduit and the return conduit, depending on the desired pressure variation.
A 3-way/3-position valve is of a complex design. It has been previously suggested to provide only one electromagnetically operable outlet valve that blocks or releases the return conduit, and to insert a restriction valve into the brake conduit so that the pump, through a diaphragm, supplies pressure fluid to the wheel brake. In that configuration, the pressure in the wheel brake can be decreased or increased, depending on whether the amount of pressure fluid per unit of time flowing out through the outlet valve is greater than that flowing in through the restriction valve. According to the state-of-the-art design, the restriction valve is provided in the brake conduit to operate also during a non-controlled brake operation. Admittedly, it has already been suggested to design the throttle switchable in order to be introduced into the brake conduit only during a control operation. However, a design of this type necessarily involves substantial mechanical efforts because corresponding switch means are required.
It is an object of the invention to design the valve switching mechanism with the throttle rigidly installed within the system so that no additional switching means are required.
It is, therefore, suggested by the invention to provide a throttle in the pressure conduit of the pump, i.e. external to the brake conduit.
It is another object of the invention to provide means enabling additional pressure fluid to be introduced into the control circuit below the separating valve. This is accomplished by connecting the master brake cylinder and the pressure side of the pump through a check valve opening toward the pressure side of the pump.
The restriction valve may comprise a firmly adjusted diaphragm of constant cross-section or a flow control valve. A flow control valve is preferred because a predetermined pressure could be adjusted on the outlet of the pump by adapting the delivery capacity of the pump to the control pattern of the flow control valve.
The separating valve can be actuated electromagnetically and hydraulically. Hydraulic actuation can be performed in various ways. For example, the separating valve can be actuated by the pressure on the output of the pump or through a plunger secured to or in abutment with the piston of the low pressure accumulator.
Moreover, it may be feasible to provide an electromagnetically actuable inlet valve in addition to the restriction valve. Such an inlet valve would be required to switch only in exceptional situations because a restriction valve is also provided. Such an exceptional situation would occur, for example, if the pressure in the wheel brake needed to be substantially lowered. In that case it would be necessary to completely eliminate the pressure supply through the restriction valve.
These and other features of the present invention can best be understood from the following specification and drawings, of which the following is a brief description.